As is generally the case in a timepiece that combines a chronograph and a watch, the gear train driving the chronograph is derived from the gear train driving the watch, and the timepiece has only a single energy source and a single resonator. By way of concrete example, cited in the work of B. Humbert “Le Chronographe” by Scriptar SA, La Conversion, (Switzerland), 1990, the top pivot of the fourth wheel set of the watch is extended to fit thereon a wheel with a contraet toothing, or a first chronograph drive wheel. The latter wheel is permanently meshed with an intermediate wheel secured to a coupling lever controlled by a column wheel, which is in turn controlled by a push button. When the lever is actuated, the intermediate wheel, also called the coupling wheel, drives a chronograph wheel set secured to a direct drive seconds-hand that completes one revolution per minute.
It will be clear that the method that has just been briefly described leads to instability in the time displayed by the watch when the chronograph is working. This instability is caused by a loss of torque causing a decrease in the impulse energy transmitted to the sprung balance. Thus, the working precision of the watch is affected when the chronograph is working.
A solution has been proposed to avoid the aforecited drawbacks. EP Patent No. 1333345 proposes a device comprising a watch whose time indicators are driven by a first barrel connected to a first gear train and a first regulating member, and an autonomous chronograph module whose indicators are driven by a second barrel, which is independent from the first, connected to a second gear train and a second regulating member.